The present invention relates to wireless communication systems. More specifically, the present invention relates to cell search in time division duplex communication systems using code division multiple access.
Cell search is a process where a wireless user, user equipment (UE 10), synchronizes with a base station of a cell prior to transmission of traffic data, such as voice data. FIG. 1 is an illustration of a UE 10 in a wireless communication system. Upon activation, the UE 10 is unaware of its location. The UE 10 selects a cell 121 out of a plurality of cells 121 to 12n(12) and its associated base station 141 for communication. Prior to initiating communication, the UE 10 synchronizes both timing and code set to the selected base station 141. 
The cell search process involves three steps. In the first step (step 1), the UE 10 identifies nearby base stations 141 to 14n(14). Each base station 14 transmits a primary synchronization code (PSC) in a primary synchronization channel (PSCH).
In a typical time division duplex (TDD) communication system using code division multiple access (CDMA), the PSCH resides in either one timeslot, for case one, or two timeslots for case two, of a fifteen timeslot frame. In case one, the PSCH is transmitted in a timeslot K out of the fifteen timeslots. In case two, the PSCH is transmitted in one of two timeslots, K and K+8. To distinguish between the different base stations 14, each base station 14 transmits its PSC in the PSCH with a particular time offset, tOFFSET, from the timeslot boundary.
In step 1, the UE 10 looks for transmitted PSCs from the base stations 14. The UE 10 looks at the one or two PSCH timeslots for received PSCs. Since the PSC is typically unmodulated, such as an unmodulated 256 chips, the search can be performed by looking for peaks in a match filter output over the timeslot(s). Each peak is a potential candidate base station 14 for synchronization.
In step 2, information about each cell is determined, such as the cell's code set, tOFFSET, the frame index number within the interleaving period of two frames and the timeslot of the cell's transmitted PSC (for case 2). To determine each cell's information, the UE 10 looks for transmitted secondary synchronization codes (SSCs) transmitted along with each PSC. At each peak, the UE 10 searches for transmitted SSCs. The SSCs may be modulated with data. Based on each base station's detected SSCs and the modulated data, the UE 10 ascertains the cell information.
In step 3, the UE 10 determines the base station 141 for synchronization. For this determination, the UE 10 monitors the broadcast channel (BCH) of the primary common control physical channel (P-CCPCH) for transmitted midambles for each of the potential base stations 14 for synchronization. The base station 141 having the midamble with the highest received power is selected as the base station 14 for synchronization.
To implement this cell search procedure, a tradeoff between complexity and synchronization time exists. To speed up synchronization, matched filters searching all the differing cells' SSCs and midambles may be used. Alternately, fewer reconfigurable matched filters may be used over multiple frames. Using multiple frames, increases the synchronization time but reduces the number of matched filters and other cell search components.
Accordingly, it is desirable to have alternate approaches for cell search.